<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script src="jquery.3.4.1.js"></script>

</head>
<body>
<input type="text">
<select name="" id="">
    <option value="">北京</option>
    <option value="">上海</option>
</select>
</body>
<script>
    $('input').focus(
        function () {
            console.log('获得焦点了')
        }
    )
    $('input').blur(
        function () {
            console.log('失去焦点了')
        }
    )
    $('select').change(
        function () {
            console.log('内容改变了')
        }
    )
</script>
</html>